The present invention relates to a semiconductor device and in particular, to a semiconductor device that constitutes an insulated gate bipolar transistor.
Insulated gate bipolar transistor (hereinafter referred to as IGBT), specifically devices which have planar gate structures as shown in FIG. 25 and FIG. 26 and devices which have trench gate structures as shown in FIG. 27 and FIG. 28, are known. A non-punch-through type IGBT as shown in FIG. 25 or FIG. 27 comprises a base layer 2 that is composed of an n-type semiconductor substrate made of for example an FZ wafer, a p-type channel diffusion region 3, an n-type emitter diffusion region 4, an emitter electrode 5, a gate-insulating film 6, a gate electrode 7, and an insulator film 8, which are formed on one surface side of the substrate, and a p-type collector layer 9 and a collector electrode 10, which are formed on a reverse surface side of the substrate.
A punch-through type IGBT as shown in FIG. 26 and FIG. 28 employs a wafer, that is an epitaxial wafer, and comprises a p-type wafer 11, an n-type semiconductor layer 12, and another n-type semiconductor layer 13 having the impurity concentration lower than that of the n-type semiconductor layer 12, the both n-type semiconductor layers being epitaxially grown on the p-type wafer 11. The body of the p-type wafer 11 constitutes a collector layer 9; the n-type semiconductor layer 12 on the collector layer constitutes a buffer layer 14; and the n-type semiconductor layer 13 on the buffer layer constitutes a base layer 2.
In the surface region on the side of the base layer 2 of the epitaxial wafer, formed are a p-type channel diffusion region 3, an n-type emitter diffusion region 4, an emitter electrode 5, a gate-insulating film 6, a gate electrode 7, and an insulator film 8. A collector electrode 10 is formed on the surface of the side of the collector layer 9, which is the reverse side of the epitaxial wafer.
However, the non-punch-through type IGBT mentioned above has a disadvantage of large loss because of the thick base layer 2, which is required so that the depletion layer in the turn-off operation does not extend beyond the thickness of the base layer 2. In the punch-through type IGBT that is also mentioned above, the thickness of the base layer 2 is about 120 xcexcm for an example of a blocking voltage class of 1,200 V. The thickness value is smaller than the thickness of about 180 xcexcm of the base layer of a non-punch-through type IGBT, which results in a lower loss of the punch-through type IGBT. However, the punch-through type IGBT has a disadvantage of its higher cost of the chip caused by the lower yield of the chip and the higher cost (over twice) of the epitaxial wafer than the FZ wafer.
In view of the foregoing, it would be desirable to provide a semiconductor device constituting an IGBT that can be produced with a high yield using an inexpensive wafer and generates little loss.
The present invention is directed to a semiconductor device, which constitutes an IGBT produced by using a wafer, for example a FZ wafer, that is cut from an ingot and having its surface polished and cleaned, wherein an n-type impurity diffusion layer having an enough dose to stop the electric field during turn-off is provided between a collector layer and a base layer as a semiconductor layer for stopping an electric field during turn-off operation (hereinafter this semiconductor layer is referred to as a field-stop layer).
The thickness of this field-stop layer defined by Xfsxe2x88x92Xj is from 0.5 xcexcm to 3 xcexcm, where Xfs is the position at which the impurity concentration in the field-stop layer becomes twice the impurity concentration of the base layer, and Xj is the position of the junction between the field-stop layer and the collector layer. One reason why the thickness of the field-stop layer, Xfsxe2x88x92Xj, is in the above indicated range is that when forming the field-stop layer by means of ion implantation, the maximum depth is 3 xcexcm due to the energy limit of the ion implantation available for practical mass-production at present. On the other hand, the reason for the lower limit is that a diffusion layer thinner than the above-indicated lower limit is difficult to be formed by ion implantation with precise control.
The voltage at which the base layer depletes completely is appropriate to be 0.45 to 0.7 times blocking voltage of this IGBT. This is because when the depletion voltage is less than the lower limit, the spike voltage at the switching operation would be close to the blocking voltage and may cause failure of the IGBT and/or failure or malfunction of the system containing the IGBT. On the other hand, if the upper limit is exceeded, power loss reduction more than 20 percent can not be expected. Generally, a new product needs reduction in power loss over 20 percent in comparison with a conventional product so as to replace the conventional one.
The peak value of the impurity concentration in the collector layer is preferably larger than 15 times the peak value of the impurity concentration in the field-stop layer. If not larger than 15 times, the on-state voltage drop exceeds 3 V, which is not practical.
The impurity concentration at the junction position between the collector layer and the field-stop layer is preferably not less than 4xc3x971016 cmxe2x88x923. If less than 4xc3x971016 cmxe2x88x923, the blocking voltage is insufficient in the case the collector potential becomes lower than the emitter potential in the actual operation.
The average donor concentration in the field-stop layer is preferably not less than 1xc3x971015 cmxe2x88x923 and not less than 15 times the donor concentration of the base layer. In an IGBT, the base layer of which withstands 600 V, for example, the field-stop layer with donor concentration less than 1xc3x971015 cmxe2x88x923 cannot withstand 600 V because the depth Xfsxe2x88x92Xj of the filed-stopping layer formed by ion implantation is limited to 3 xcexcm; that is, the IGBT cannot perform the blocking voltage of 1,200 V. The average donor concentration of the field-stop layer is not less than about 15 times the donor concentration of the base layer with resistivity of 60 xcexa9cm, the latter concentration being 7xc3x971013 cmxe2x88x923.
Appropriate amount of dose to the field-stop layer is from 3xc3x971011 cmxe2x88x922 to 1xc3x971012 cmxe2x88x922. This is because the average donor concentration is not less than 1xc3x971015 cmxe2x88x923 and the dose of the field-stop layer having thickness Xfsxe2x88x92Xj of 3 xcexcm is not less than 3xc3x971011 cmxe2x88x922. When the thickness Xfsxe2x88x92Xj of the field-stop layer is 0.5 xcexcm, the field-stop layer with the average donor concentration of 2xc3x971016 cmxe2x88x923 may be supposed to withstand 600 V. The dose corresponding to this donor concentration is 1xc3x971012 cmxe2x88x922, which becomes an upper limit of the dose.
Advantageously, the voltage VA at which the field-stop layer and the base layer set up punch-through at 25xc2x0 C. is larger than 1.54 times the voltage VB that is determined by the avalanche breakdown of the pn junction or smaller than 0.84 times the voltage VB. While a temperature range for performance guarantee of an IGBTs is generally from xe2x88x9220xc2x0 C. to 150xc2x0 C., if the ratio of VA to VB at 25xc2x0 C. is in either of the above-specified ranges, VA and VB does not approach too close, as a result, failure of the IGBT hardly occurs in the whole temperature range from xe2x88x9220xc2x0 C. to 150xc2x0 C.
According to the present invention, an impurity diffusion layer that performs as a field-stop layer is formed in the region of one principal surface of a semiconductor substrate with the depth of not more than 3 xcexcm, which is the maximum depth capable of implanting within the practical limit of ion implantation energy. Because this impurity diffusion region can be formed by means of an ion implantation, an IGBT can be produced with high yield using an inexpensive wafer such as an FZ wafer, same as in a non-punch-through type IGBT. Moreover, the field-stop layer allows the base layer to be as thin as that of the punch-through type IGBT, which achieves low loss.